<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<script src = "js/jquery-3.4.1.min.js"></script>
	<script>

	$(function(){

		// 1 是选中 2是未选中
		$div1 = $('#div1');
		
		alert($div1.length);

		$div2 = $('#div2')
		alert($div2.length)
	})

	</script>

</head>
<body>
	<div id="div1">这是一个div1</div>

</body>
</html>